In his famous Summa Theologica, the Scholastic theologian Thomas Aquinas presents Five Ways to demonstrate the existence of God. Here is Aquinas' First Way, the argument from motion:
The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.
Here is the basic logic of the argument as I understand it:
Motion, defined as the actualization of a potential attribute of an object, exists.
The actualization of a potential attribute of an object cannot be caused by that attribute, because only what is actual can cause what is potential to become actual.
Therefore, the motion of an object must be caused by some other object.
If that other object is itself undergoing motion with respect to some attribute, this motion must be caused by some yet another object, and if that object is in motion then that motion must be caused by the still another object, etc.
The immediate efficient cause of something is simultaneous with it, so we have an essentially ordered series of causes existing in the present moment. But essentially ordered causal series cannot go on forever.
Thus the sequence must end with an object which is not being moved with respect to any of its attributes, an unmoved mover. And Aquinas defines this as God.
My question is about the reasoning involved in going from step 2 to step 3. What is the justification for step 3? Why can't the actualization of a potential attribute of an object be caused by an actual attribute of the same object? If that were the case, then I don't see how the infinite regress argument could possibly go through.
Any help would be greatly appreciated.
Thank You in Advance.
EDIT: My understanding of Aquinas' First Way is based on the works of Edward Feser. Here is a discussion of the First Way from his book The Last Superstition.